1. Field of the Invention
The invention relates to a tire composition with improved abrasion performance and fatigue resistance, in particular for tires.
2. The Prior Art
A very wide variety of additives is admixed with the mixtures in order to influence the properties of the mixture and of the vulcanizate, and/or specific polymers are used for this purpose. Examples that may be mentioned here of additives are fillers (e.g. carbon black), plasticizers, antioxidants, and crosslinking systems composed of sulfur, accelerators, and activators. However, if one property is improved by varying the mixture, this is often attended by impairment of another property, and there are therefore certain conflicts of objectives. Examples of these conflicting objectives in the case of mixtures for tire treads are found in relation to abrasion performance, fatigue resistance and increased build-up of heat, which causes poorer rebound resilience and therefore poorer rolling resistance. A particular method used to solve these conflicts of objectives is variations in the constitution of the mixture, and also in particular changes or modification in additives, the aim being to achieve an improved level of properties which are usually inversely correlated.
An important group of additives which influences vulcanization rate and the physical properties of the vulcanizates is the group of the vulcanization accelerators. There are various groups of vulcanization accelerators available for production of tires and known to the person skilled in the art, and these can also be used in combination with one another, sometimes giving synergistic effects.
These vulcanization accelerators serve for activation of the sulfur used as vulcanizing agent. The addition of sulfur and vulcanization accelerator here is individually matched to the tire-rubber-mixture properties to be achieved. These properties to be achieved are a function of the network produced during vulcanization, e.g. between polymer chains itself or polymers and fillers, and great importance can therefore be attached to the nature and the degree of crosslinking with a view to the physical properties of the vulcanizates.
Special importance is the structure of the crosslink, well-known for those skilled in the art. S-length distribution influences fatigue behavior.
The prior art in relation to vulcanization systems or crosslinking systems will now be described in more detail, using the following publications:    (D1) DE 603 03 203 T2    (D2) U.S. Pat. No. 5,342,900    (D3) US 2002/0058760A1    (D4) EP 0 530 590 B1    (D5) U.S. Pat. No. 7,189,866
D1 discloses a polysulfide siloxane that can be used as crosslinking agent, and the process for its preparation. The crosslinking system here encompasses the polysulfide siloxane described and at least one primary vulcanization accelerator. The polysulfide siloxane is used in a composition based on a diene elastomer and on a reinforcing filler. The diene elastomer described here comprises various components and the fillers described here comprise in particular silica and carbon black, and each of the examples disclosed here relates to a rubber mixture composed of natural rubber as single polymer and carbon black as single filler.
D2 discloses a vulcanized diene rubber wherein the vulcanizing is carried out in the presence of a cross linker containing benzyl groups, sulfur and mercapto accelerator and a sulfenamide accelerator.
D3 discloses a molding method for protective equipment, particularly one capable of sufficiently connecting a plastic plate and a foam member of a piece of protective equipment and molding both in shape at the same time without any adhesive periphery.
D4 discloses a process for the production of diene rubber vulcanizates with very high aging resistance and reversion resistance. The diene rubber vulcanizates here comprise from 1 to 2.5 parts of mercapto accelerator or from 0.2 to 0.8 part of sulfenamide accelerator, or from 0.3 to 2.5 parts of mercapto accelerator and from 0.1 to 0.8 part of sulfenamide accelerator. From 0.1 to 0.2 part of sulfur is also used per 100 parts of rubber, preferably of an oil-extended diene rubber.
D5 relates to cross-linking agents usable for cross-linking elastomeric networks, in particular in the manufacture of tires or semi-finished products for tires. In the examples the process is carried out with the cyclic polysulfurized tetramethyldisiloxane. This process shows that it is possible to cross-link without the addition of sulfur, a rubber composition. Also demonstrated is improvement in the thermal stability (reversion behaviour) of the compositions based on the polysulfide.
The disclosure of each of the above prior art documents is herewith incorporated by reference.
Elemental sulfur is commonly used as a vulcanizing agent for unsaturated organic polymers. The crosslinks that sulfur forms with the organic polymer are primarily polysulfidic crosslinks that reduce the thermal stability of the vulcanizates. The use of organic compounds that have sulfur containing reactive groups is known in the art as vulcanizing agents for diene rubbers. These sulfur-containing compounds often contain only two dithiocarbamate or sodium thiosulphonate groups chemically bonded to a bridging group. The low number of tie points are ineffective at crosslinking the unsaturated diene polymers and achieve vulcanizates with a good balance of wear and tear resistance. It would be desirable to have a new type of crosslinker for elastomers that would improved fatique, tear and wear properties, especially tear and abrasive wear, while maintaining hardness.
Usually rubber mixtures are vulcanized with crosslinkers providing two tie points, which, according to theory, have a crosslink functionality of 4, which means that 4 polymer arms are linked with the crosslink. In the process the crosslink-density in crosslinked rubber mixture is approximately between 10·10−5 to 25·10−5 mol/cm3 (determination by equilibrium swelling in unfilled rubber compounds).